You are right when you are talking about a browser automatically sending a file without user intervention. Browsers shouldn't do that.

But, the problem posed here is about having a LWP::UserAgent script submit a form, not about a browser doing it without a user's consent. Submitting a file with a request is perfectly doable with LWP::UserAgent.

A quick search at google for "upload file script LWP" pointed me to this URL with an example script for doing this.